1. Field of the invention
The present invention relates to refrigerating apparatus, and in particular, to an improvement in a multiple-type refrigerating apparatus for an air-conditioner, freezer or display case.
2. Description of the prior art
A well known prior art refrigerating apparatus, a so-called multiple-compressor refrigerator, is shown in FIG. 1. The exhaust-sides of each of a plurality of compressors 1, 3 and 5 are communicated to parallel connected condensers 7, 9 and 11 through a refrigerant discharge pipe 13. The output sides of condensers 7, 9 and 11 are collectively connected to parallel connected electromagnetic valves 15, 17 and 19, through a refrigerant pipe 21 which includes a capillary element (not shown). Electromagnetic valves 15, 17 and 19 are connected to an accumulator 23 through individual evaporators 25, 27 and 29. Electromagnetic valves 15, 17 and 19 include individual thermostats (not shown) which are provided on evaporators 25, 27 and 29 respectively. When the temperatures of evaporators 25, 27, or 29 reach a predetermined low temperature, a corresponding electromagnetic valve 15, 17 or 19 is closed automatically. Accumulator 23 is connected to the suction sides of compressors 1, 3 and 5 through a refrigerant intake pipe 31.
A pressure sensor 33, communicated to refrigerant intake pipe 31, is electrically connected to an electronic control circuit 35. Pressure sensor 33 detects refrigerant suction pressure of the suction sides, i.e. low pressure side, of the compressors. Electronic control circuit 35 is connected to magnetic switches 37, 39 and 41 which are individually provided on compressors 1, 3 and 5. Magnetic switches 37, 39 and 41 turn corresponding compressors 1, 3 and 5 on and off respectively in response to the output signal of electronic control circuit 35.
The operation of the above-described refrigerating cycle will be described hereinafter. When compressors 1, 3, and 5 are actuated, refrigerant circulates through evaporators 25, 27 and 29. At this moment, compressors 1, 3 and 5 are actuated at the same operating frequency and at the same rotary speed. Respective evaporators 25, 27 and 29 therefore operate identically. If the temperature of one of evaporators, e.g. evaporator 25, falls below a prescribed temperature due to its refrigeration load or other environmental condition, the corresponding electromagnetic valve 15 is closed and the refrigerant flowing into evaporator 25 is stopped. The low pressure side of the refrigerant (the suction sides of the compressors) therefore falls below a prescribed value and pressure sensor 33 detects this pressure fall and sends a signal to electronic control circuit 35. The electronic control circuit compares the signal with a set value to control respective compressors 1, 3 and 5. That is, one of compressors 1, 3 and 5 is stopped and the others are operated. The amount of refrigerant circulated is reduced, the pressure of the suction side of the compressors gradually rises and the refrigerating cycle suited to the number of evaporators which have been opened by the electromagnetic valves is effective.
In this arrangement, a variable refrigerating cycle is provided by a plurality of compressors and condensors corresponding to the number of the compressors. Thus, installation space and operating costs are very large. There is also the disadvantage that it is not always possible to achieve optimum conditions because it is only possible to carry out stepwise control in correspondence to the number of compressors.
It has been proposed to apply an analogue control system, a so-called inverter control system, in which the operating frequency of a single compressor is controlled by an inverter circuit and thus achieve optimum operating conditions.
According to this control system, a single compressor, and a common condensor can be used to reduce installation space and manufacturing costs. It further has another advantage that the optimum control is possible because of stepless frequency control. However, the control section of the inverter circuit receives signals from a pressure sensor which detects the pressure side (suction side) of the compressor as in a conventional system. There are many errors in pressure measurement, and sensors in which errors are reduced are extremely expensive. These improved sensors have an adverse effect on costs. The detection of pressure of the low pressure side means detecting the pressure at the return side of a refrigerating cycle, i.e. the pressure of the refrigerant just before return to the compressor. Response is slow and it is difficult to detect slight changes of pressure.